Before a single bench is cut, a mine is planned from the inside out. Geologists drill and sample the ground on a grid, lay the cores out in trays, and read them like pages of a book. In a lateritic bauxite deposit, the story is told in five chapters.
Chapter one: topsoil
The top of the core is dark, organic soil with roots, typically the first metre or two. It has no ore value, but it is one of the most valuable materials on site, because it holds the seeds, nutrients and soil life that reclamation depends on. Good practice is to strip and bank it before mining, then spread it back over refilled ground. See mine reclamation.
Chapter two: the laterite cap
Below the soil comes a hard, rusty, often pisolitic or vesicular crust rich in iron. This is the laterite cap, formed where iron oxides concentrated near the surface. It is usually overburden: it must be removed to reach the ore, and its thickness, together with that of the topsoil, decides the stripping ratio, the tonnes of waste moved for each tonne of ore.
Chapter three: the bauxite horizon
Then the colour lightens to cream, pink, buff or ochre. This is the bauxite zone, where weathering has leached silica away and concentrated aluminium as gibbsite. The core may be massive or porous, sometimes with relict textures of the original basalt.
Here the logging becomes detailed. Each interval is sampled and assayed for Al₂O₃, SiO₂, Fe₂O₃, TiO₂ and LOI, so the geologist can see not only where the ore is, but which grade it is. A high-alumina, low-iron stretch might be refractory material; a lower-iron, low-reactive-silica stretch suits a refinery.
Chapter four: lithomarge
Towards the bottom of the ore, the core turns soft, smooth and pale or mottled purple. This is lithomarge, a kaolin-rich clay. It is high in silica, especially reactive silica, and usually marks the floor of mining. Mining too deep dilutes good ore with clay; stopping too high leaves ore behind. See reactive silica in bauxite.
Chapter five: basalt
At the base lies grey, partly weathered and then fresh basalt, the Deccan Trap lava from which everything above was formed.
An illustrative profile
The profile below is the illustrative section used on our home page's drill animation. Real thicknesses change from one drill hole to the next.
| Depth (m) | Layer | Role in mining |
|---|---|---|
| 0 – 1.5 | Topsoil | Stripped and banked |
| 1.5 – 8 | Laterite cap | Overburden |
| 8 – 17 | Bauxite ore | Mined, crushed, screened and graded |
| 17 – 21.5 | Lithomarge | Floor of mining |
| 21.5 – 30 | Basalt | Parent rock |
From cores to a mine plan
Once enough holes are drilled, the logs and assays are combined into a model of the deposit: where the ore is thickest, where the cap is thinnest, and where each grade sits. That model guides:
- Where to open the first benches, and in what sequence.
- How deep each bench should go before reaching lithomarge.
- Which material is routed to which grade stockpile.
- Where waste and topsoil will be stored for backfilling.
This is the first of the six steps in our pit-to-port process, and the reason the rest of the process works.